Why did Trotsky become more unpopular from 1924?

In 1924 Trotsky launched an attack on the lack of Party democracy. Centralisation had occurred during the Civil War, so Trotsky's attack appeared to be an attack on something Lenin had sanctioned.

The central bureaucracy also provided jobs for many members, so Trotsky's attack was not likely to be popular.

October 1924 Trotsky attacked Zinoviev and Kamenev in an essay, Lessons of October. It drew attention to the fact these two had opposed Lenin when the decision to launch the October Revolution was made.

This led to a war of words Kamenev pointed to Trotsky's Menshevik past. Stalin remained aloof and watched his opponents destroying each other. Stalin appeared to be moderate who was 'above' Party warfare.